Dielectric relaxation study of N -methyl formamide with glycols using time domain reflectometry technique

The dielectric relaxation study that is static dielectric permittivity (∈₀) and relaxation time (τ) of amide of N-methyl formamide (NMF) with increasing volume percent propylene glycol (PLG) and BLG has been carried out at different temperatures. The time domain reflectometry (TDR) technique has been used to measure reflection coefficient in frequency range of 10 MHz to 20 GHz. The dielectric parameters have been obtained by fitting experimental data with the Havriliak–Negami equation. The experimental observation shows that the static dielectric permittivity and relaxation time decreases with increasing temperature. The experimental observation also shows that the static dielectric permittivity decreases and relaxation time increases with increasing percentage volume of Propylene glycol (PLG) and Butylene glycol (BLG) in NMF. The nature of (?₀) and (τ) is same for the temperature ranges (20, 30, and 40°C). The thermodynamic parameters enthalpy (ΔH) and entropy (ΔS) of the binary mixture are also reported in this work.     

Dielectric relaxation of butyl acrylate—alcohol mixtures using time domain reflectometry

Dielectric relaxation measurements of butyl acrylate—alcohol mixtures at different concentrations and temperatures within the frequency range of 10 MHz to 10 GHz have been carried out using time domain reflectometry. Parameters such as the static permittivity, dielectric relaxation time, the Kirkwood correlation factor, the excess inverse relaxation time, and thermodynamic functions were determined and discussed to yield information on the molecular structure and dynamics of the mixture. The value of the dielectric properties decreases with increasing butyl acrylate concentration in alcohol and systematically varies with the length of alcohol alkyl chain. Negative values of the excess inverse relaxation time found for all concentrations and at all temperatures studied may indicate that the effective dipoles rotate slowly.     

Dielectric relaxation studies of aqueous N,N-dimethylformamide using a picosecond time domain technique

The dielectric relaxation studies of N,N-dimethylformamide at thirteen concentration in aqueous solutions have been carried out using a time domain reflectometry technique in the frequency range 10 MHz to 10 GHz. The dielectric parameters and excess dielectric properties have also been determined from 5 to 40°C. The Luzar theory was applied to compute the cross correlation terms for the mixture. It adequately reproduces the experimental values of the static dielectric constants. The Bruggeman model for the nonlinear case has been fitted to the dielectric data for mixtures.     

Broadband dielectric relaxation study on a diluted solution of a ferroelectric liquid crystal by time domain reflectometry

Abstract

In this work, complex permittivity measurements on a diluted solution of the ferroelectric liquid crystal, 4-[(S,S)-2,3-epoxyhexyloxy]-phenyl 4-(decyloxy)-benzoate, which shows ferroelectric smectic C phase (S*_C), have been performed. Using time domain reflectometry, at frequencies between 10 MHz and 10 GHz, in the temperature range from 50°C down to 10°C for every 10°C, at 5, 10 and 30 wt.% in benzene we observe two independent relaxation processes around 150 MHz and 2 GHz. It is shown that the high frequency process is due to the internal molecular reorientations while the low frequency one is due to molecular orientation around the long molecular axis. It is concluded that the reorientation around the long axis is hindered and leads to a resultant macroscopic polarization.     

Broadband dielectric relaxation study on a diluted solution of a ferroelectric liquid crystal by time domain reflectometry

In this work, complex permittivity measurements on a diluted solution of the ferroelectric liquid crystal, 4-[(S,S)-2,3-epoxyhexyloxy]-phenyl 4-(decyloxy)-benzoate, which shows ferroelectric smectic C phase (S*_C), have been performed. Using time domain reflectometry, at frequencies between 10 MHz and 10 GHz, in the temperature range from 50°C down to 10°C for every 10°C, at 5, 10 and 30 wt.% in benzene we observe two independent relaxation processes around 150 MHz and 2 GHz. It is shown that the high frequency process is due to the internal molecular reorientations while the low frequency one is due to molecular orientation around the long molecular axis. It is concluded that the reorientation around the long axis is hindered and leads to a resultant macroscopic polarization.     

Numerical and approximate studies on the theoretical dielectric relaxation of colloidal suspensions in the time domain

In this work we show numerical calculations on the dielectric behavior of colloidal suspensions in the time domain. The theory elaborated by DeLacey and White ((1981) J Chem Soc Faraday Trans 2 77:2007–2039) for dilute suspensions in the frequency domain, will be the basis for the present study. The different contributions, and their relative importance, to the transient current density generated in the suspensions after the application of a step electric field, are calculated from the dielectric response function associated to the DeLacey and White's model. In particular, we analyze the conduction and absorption current densities in the transient states upon changing the concentration of the supporting electrolyte in the suspension. With the aim of characterizing the response of the suspension for short times, an approximation to the distribution function of relaxation times that best fits the dielectric model, is calculated. Finally, an exhaustive analysis of the behavior of the dielectric response function is carried out, together with a comparison with other models in the time domain.     

Effect of a third component on the interactions in a binary mixture determined from the fluctuation theory of solutions

The Kirkwood–Buff (KB) theory of solution is applied to a ternary mixture by deriving explicit expressions for the various Kirkwood–Buff integrals (KBIs) and the corresponding excesses of the number of molecules around central ones. However, the ideal solution should be considered non-aggregated, and the above expressions for the excesses provide non-zero values for such a case. For this reason, in order to obtain information about clustering one must subtract from the traditional excesses those which correspond to a reference state, thus ensuring that for an ideal mixture the excesses are zero. The expressions derived for the latter excesses have been applied to the investigation of the N,N-dimethylformamide–methanol–water mixture, to conclude that: (i) in the vicinity of the water molecules there are excesses of water and N,N-dimethylformamide molecules and a deficit of methanol molecules; (ii) in the vicinity of the methanol molecules there are excesses of methanol and N,N-dimethylformamide molecules and a deficit of water molecules; (iii) in the vicinity of the N,N-dimethylformamide molecules there are excesses of methanol and water molecules and a deficit of N,N-dimethylformamide molecules; (iiii) the excesses of N,N-dimethylformamide around water and methanol molecules and those around N,N-dimethylformamide are weakly dependent on the concentration of the third component in a large range of concentrations of the latter, and these results are compatible with the existence of N,N-dimethylformamide–water and N,N-dimethylformamide–alcohol complexes.     

Dynamic structure of poly(vinyl pyrrolidone)/ethyl alcohol mixtures studied by time domain reflectometry

Dielectric studies of poly(vinyl pyrrolidone)/ethyl alcohol (PVP–E) binary mixtures with concentration variations were carried out in the frequency range of 10 MHz to 10 GHz by time domain reflectometry at 15, 25, 35, and 45 °C. One relaxation process, corresponding to ethyl alcohol molecules in the poly(vinyl pyrrolidone) (PVP) matrix, was observed in this frequency range for all the mixtures. The static dielectric constant of the PVP–E mixtures decreased linearly with an increase in the weight fraction of PVP. The observed anomalous increase in the value of the relaxation time (τ) of these mixtures was interpreted by the consideration of the variation in the local structure of self‐associated ethyl alcohol molecules and also the PVP behavior as a geometric constraint for the rotational motion of ethyl alcohol molecules. Furthermore, the τ values of these mixtures were independent of the viscosity. The energy parameters for the dielectric relaxation process (the free energy, enthalpy, and entropy of activation for the dipolar orientation) were determined to confirm the transient behavior of the heterogeneous species due to the breaking and re‐forming of hydrogen bonds with the internal rotation of ? OH groups in the ordered structure of the PVP–E mixtures. On the basis of the evaluated dielectric parameters, the formation of supermolecular structure in the PVP–E mixtures in dynamic equilibrium was sketched and examined by the consideration of the hydrogen bonding between the terminal hydroxyl groups of self‐associated ethyl alcohol flexible chains and the carbonyl groups of monomer units of PVP coiled chains. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1134–1143, 2005     

On the visco-elastic and dielectric behaviour of two alcohols and their binary mixture

Inter-molecular and intra-molecular interactions in liquids determine the physical properties of the systems. These interactions are understood through the measurement of these physical properties. These become especially important in the case of alcohols in view of the specific type of interactions involved. Study of the variation of dielectric relaxation time with the viscosity of the medium is relevant in drawing certain quantitative conclusions regarding molecular motion and the inter-molecular forces in liquids, liquid mixtures, dilute solutions and multi-component polar solutes in dilute solution. In the absence of a perfect empirical or theoretical equation for the variation of dielectric relaxation time with viscosity, the experimental investigations on different systems can only give an insight. In the present study, the results of dielectric measurements carried out on pure samples of methyl alcohol and propyl alcohol in dilute solutions in different mixed solvents (benzene?+?paraffin) and on binary mixture (1?:?1) of methyl alcohol?+?propyl alcohol are reported. Different parameters determined are presented and these studies indicate that the dielectric behaviour at microwave frequencies favour the concept of dynamic viscosity and a single visco-elastic relaxation time for the systems under study.     

Dielectric relaxation and hydrogen bonding studies of chlorobutane-dioxane mixtures using a time domain technique

Complex permittivity spectra have been computed for the binary mixtures of Chlorobutane (CLB) with 1, 4-Dioxane (DX) using Time Domain Reflectometry (TDR) for different concentrations and temperatures in the frequency range from 10 MHz to 30 GHz. The static dielectric permittivity and relaxation time have been obtained by fitting complex permittivity spectra to the Debye relaxation using least squares fit method. The Kirkwood correlation factor have been determined at various concentrations of 1, 4-dioxane. The Bruggeman model for the non-linear case has been fitted to the dielectric data for the mixtures.     

Dielectric relaxation study of aniline,N-methylaniline and N,N-dimethylaniline and alcohol in 1, 4-dioxane using picosecond time-domain reflectometry

A time-domain reflectometry technique has been used to measure complex dielectric permittivity ε*(ω) = ε?(ω) ? jε″(ω) of 1-propanol–dioxane, 2-propanol–dioxane, aniline–dioxane, N-methylaniline–dioxane and N,N-dimethylaniline–dioxane mixtures in the frequency range of 10 MHz to 30 GHz. The complex permittivity spectrum has been fitted with a single relaxation time with a small amount of Davidson–Cole behaviour. The least squares fit method has been used to obtain the static dielectric constant (ε₀), relaxation time (τ), Bruggeman factor and Kirkwood correlation factor. The Luzar theoretical model is used to compute the binding energies and average number of hydrogen bond between co-solvent–co-solvent and co-solvent–dioxane molecules.     

H-bonded molecular interaction study on binary mixtures of mono alkyl ethers of ethylene glycol with different polar solvents by concentration dependent dielectric analysis

Static dielectric constant measurements on binary mixtures of the homologous series of mono alkyl ethers of ethylene glycol with six different characteristic polar solvents i.e. ethyl alcohol, ethylene glycol, glycerol, water, dimethyl formamide and dimethyl sulphoxide over the entire concentration range were carried out using precision LCR meter and a four terminal liquid dielectric test fixture at 1?MHz and 25°C. The concentration dependent excess dielectric constant and Kirkwood correlation factor were determined for the confirmation of solvent–cosolvent heterogeneous molecular interactions. The values of stoichiometric ratio corresponding to maximum interactions between the mixtures constituents were also estimated from the concentration dependent values of ?^E. It is observed that the behaviour of heterogeneous interactions significantly varies with the increase in molecular size of the homologous series and also with the change in the mixture constituents. Comparative dielectric parameters values of the studied binary mixtures were applied to recognize the dipolar orientation due to heterogeneous interactions of mono alkyl ethers with hydroxyl group(s) containing solvents and non-hydroxyl group containing solvents.     

Understanding the efficacy of N,N-dimethylformamide and oxalyl chloride combination as chemoselective O-formylating agent: An unified experimental and theoretical study

We have developed a simple but efficient synthetic protocol for the O-formylation of a wide range of aromatic hydroxyl/phenolic substrates using an N,N-dimethylformamide (DMF) and oxalyl chloride [(COCl)₂] combination in dichloromethane (DCM) as solvent at ambient temperature. The DMF/(COCl)₂ combination was found to be highly chemoselective for the aromatic/phenolic hydroxyl group over aliphatic hydroxyl or aromatic amine/thiol groups. This chemoselectivity of DMF/(COCl)₂ combination towards O-formylation of aromatic alcohols was explained on the basis of outcomes of both experimental and density functional theory–based theoretical studies.     

On the effect of temperature on the dielectric relaxation time of some benzene derivatives and certain of their binary mixtures

Dielectric relaxation behaviour of polar molecules in a non-polar solvent, or mixtures of these substances at different microwave frequencies and over a range of temperatures and concentrations give an idea about inter- and intra-molecular forces. Also such studies enable one to calculate thermodynamic parameters such as, the change of activation energy for dipole orientation (Δ G?), the enthalpy (Δ H?) and entropy (Δ S?) of activation. Such studies in the case of binary, ternary, etc. mixtures of polar molecules in pure liquid phase or in dilute solution phase of them in a non-polar solvent help in drawing certain quantitative conclusions regarding their relaxation behaviour as to whether a single component is responsible for observed microwave absorption, or a cooperative phenomenon (average) by all the dipoles of the mixture contribute to it. An experimental investigation is here performed on typical systems. With this in view, systematic dielectric measurements in a range of temperatures are carried out at a single microwave frequency on a single weight fraction in benzene of the four substituted phenols, namely, p-fluorophenylacetonitrile, p-bromonitrobenzene, m-bromonitrobenzene and 2-chloro-6-fluoro-benzaldehyde and on binary (1?:?1) mixtures of [p-2-chloro-6-fluoro-benzaldehyde?+?o-ethylphenol] and [p-fluorophenylacetonitrile?+?2-n-butyl phenol] in benzene as solvent at different temperatures. The results are presented and discussed.     

A novel technique to measure the surface electrical potential of polymer films by using dielectric spectroscopy

A novel method that allows the determination of the electrical potential of a polymer surface has been applied for polypropylene film treated by N₂ + H₂ cold plasma. The plasma treatment results in formation on the film surface of NH₂ groups, which then gets transformed to NH₃⁺ in contact with an electrolyte and leads to the formation of an electrical double layer. The method consists of theoretical calculation of electrical model potential, using the measurement of the electrical capacitance of the film in contact with an electrolyte by dielectric spectroscopy. Comparison with the results obtained by theoretical model shows similar dependencies of the electrical potential as a function of amino‐groups density and electrolyte concentration, but systematic differences of absolute values are observed. Copyright © 2005 John Wiley & Sons, Ltd.     

Implementation of the finite field perturbation method in the CRYSTAL program for calculating the dielectric constant of periodic systems

The finite field approach has been implemented in the periodic ab initio CRYSTAL program and been used for calculating the dielectric constants of crystalline LiF and MgO (FCC structure) and BeO (wurtzite structure). To maintain the periodicity along the applied field direction, a "sawtooth" potential is used in conjunction with a supercell scheme. Supercells four to five times longer than the primitive cell in the direction of the applied field provide well-converged results. The influence of the computational parameters is discussed. An alternative scheme has also been implemented, for inner check, that consists of applying a static electric field to a slab of increasing thickness in the direction orthogonal to the surface; the dielectric response at the center of the slab is shown to converge rapidly to the bulk value evaluated with the sawtooth field. The method is accurate and permits the determination of nonlinear corrections to the dielectric constant. When used in conjunction with the local density approximation (LDA) scheme, it provides for the dielectric constant of the three above-mentioned compounds values close to those recently obtained with a time-dependent density functional theory approach.     

Solvation of Na+ in N,N-dimethylformamide + methanol solutions. A nuclear magnetic relaxation study using dynamic solvent isotope effects

Inter- and intramolecular nuclear magnetic quadrupole relaxation measurements have been used to study the system methanol (CH₃OH)+ N,N-dimethylformamide (DMF)+NaI at 25°C. The dynamic behavior of the solvent molecules was investigated, throughout the composition range of the binary mixtures, by means of ¹⁴ N relaxation of DMF and ² H of methanol-d ₁ (CH ₃ OD). The intermolecular relaxation of ²³ Na⁺ in pure DMF was used to obtain information about the symmetry of the solvent electric dipole arrangement in the solvation sphere of the ion. The investigation of preferential solvation around Na⁺ in the binary mixtures was carried out by means of ²³ Na⁺ relaxation measurements using, for the first time, both the CH ₃ OH/CD ₃ OD and the DMF/DMF-d ₇ dynamic isotope effect. The results show that, throughout the composition range, there is preferential solvation by DMF. Furthermore, the use of the isotope effects of both components allowed for the first time a basic check of the reliability of the method since we obtained two independent sets of data for the composition of the Na⁺ solvation shell in the mixtures. The consistency of the two separate data sets demonstrates that the application of the dynamic isotope effect represents a powerful tool in preferential solvation studies.     

Application of the field‐theoretic method of Bohm and Pines to a study of the metal–insulator transition in doped dielectric media

A treatment based on the field‐theoretic formalism of Bohm and Pines is presented which reproduces theoretically the essential features of the Mott–Edwards–Sienko relation, na∼¼, for the location of the metal–insulator transition in doped dielectric media, where n_c is the critical electron concentration and a is the effective radius. The model allows a study to be made of the dopant electronic wave function from the localized insulating state through to the metallic regime. The effective interparticle interaction shows Friedel oscillations and, at short range, is close to the Thomas–Fermi form. The doping dependence of the electronic hyperfine interaction, total dielectric constant, and ionization energy for a disordered collection of s‐state one‐electron atoms in a structureless dielectric medium are derived and both are found to be in satisfactory qualitative agreement with experiment. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 111–120, 1999     

A novel method to characterize thermal properties of the polymer and gas/supercritical fluid mixture using dielectric measurements

Gases or supercritical fluids (SCF) are widely used in polymer science and engineering, as their dissolution into polymeric materials will alter their inherent thermal properties; including melting and crystallization temperatures (T_m and T_c). One possible method to determine these temperatures, at elevated pressures, is to use a high-pressure differential scanning calorimeter (HP-DSC). However, the elevated pressures used in HP-DSC may result in signal instabilities, limiting the testing window for these pressures. This study presents a novel testing system using dielectric measurements to determine the effects of dissolved gas/SCF on the T_m and T_c of polymers. We have developed an instrument to determine the dielectric properties of both polymer/gas and polymer/SCF mixtures, at elevated pressures and temperatures. Using the change in the measured dielectric constant or loss, T_m and T_c were determined. The effects of hydrostatic pressure and plasticization due to dissolved carbon dioxide (CO₂) and Helium (He) on the T_m and T_c of high density polyethylene (HDPE) are presented and discussed. Both T_m and T_c increase with pressure and decrease due to plasticization, i.e., pressure and plasticization are competing variables. The dissolution of He, having a low solubility into HDPE, reveals that pressure is the dominant effect. In contrast, the dissolution of CO₂, having a high solubility into HDPE, shows that plasticization is predominant.